1. Field of the Invention
The invention relates to the field of electronics. More specifically, the invention relates to direct current buses (“DC buses”) used in power modules.
2. Description of the Related Art
An inverter power module is commonly used to convert direct current (“DC”) to alternating current (“AC”) to power a three-phase motor. The power module typically has three pairs of switches on a substrate that is secured to the module baseplate. Each switching pair has a positive or “high” side switch and a negative or “low” side switch for controlling the flow of electric current. Each switching pair is referred to herein as a “bridge,” and each half of the switching pair is referred to as a “half-bridge.” The “high side” of the bridge contains the positive switches, and the “low side” contains the negative switches. By the term “switch” is meant a switching device such as an insulated gate bipolar transistor (“IGBT”) or Metal Oxide Semiconductor (“MOS”) or Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”).
Elements may be described herein as “positive” or “negative.” An element described as “positive” is shaped and positioned to be at a higher relative voltage than elements described as “negative” when the power module is connected to a power source. “Positive” elements are positioned to have an electrical connection that is connectable to the positive terminal of a power source, while “negative” elements are positioned to have an electrical connection that is connectable to a negative terminal, or ground, of the power source. Generally, “positive” elements are located or connected to the high side of the power module and “negative” elements are located or connected to the low side of the power module.
In a typical power module configuration, the high side switches are on one side of the module opposite the corresponding low side switches. A positive DC lead from a power source such as a battery is connected to a conducting layer in the high side of the substrate. Likewise, a negative DC lead from the power source is connected to a conducting layer in the low side of the substrate. The switches control the flow of current from the conducting layers of each half bridge substrate to output leads. Output leads, called “phase terminals” transfer alternating current from the three pairs of switches to the motor.
Power modules typically have three bridges combined into a single three-phase switching module, or single half-bridge modules that may be linked together to form a three-phase switch. As would be understood by one of ordinary skill in the art, the same DC to AC conversion may be accomplished using any number of switching pairs, and each switching pair may contain any number of switches. For simplicity and clarity, all examples herein use a common three phase/three switching pair configuration. However, the invention disclosed herein may be applied to a power module having any number of switches.
Current flows from the positive DC lead to the conducting layer on the high side substrate. Current is then permitted to flow through the switching device on the high side to the conducting layer on the low side. A phase terminal lead allows current to flow from the conducting layer on the low side to the motor. The current then flows from the motor to the conducting layer on the low side of a second switching pair to the negative DC lead to the power source.
Current flowing through various paths within the module creates inductances, which in turn results in inductive power losses, reduced efficiency, and the excess generation of heat. When the flow of current changes, as in such a high frequency switching environment, large voltage overshoots often result, further decreasing switching efficiency. In addition, the DC terminals are commonly attached to one end of the power module, which forces current to travel further to some switches, and thus, for some switching configurations, than for others, resulting in non-uniform current loops. Current loops that are not uniform result in uneven or inefficient motor performance.
These and other problems are avoided and numerous advantages are provided by the device described herein.